Fuel cell power plants are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid and oxygen containing oxidant reactant streams to power electrical apparatus such as motors, and transportation vehicles, etc. In fuel cell power plants of the prior art, it has been discovered that, upon start up of fuel cells of the plant, corrosion takes place on electrodes, and especially on cathode electrodes. That corrosion leads to performance loss of the cathode electrodes and the plant.
In starting up known fuel cell power plants that contain air on both electrodes and that employ a proton exchange membrane “PEM” as an electrolyte disposed between a cathode and anode electrode, an oxygen containing oxidant is directed to flow through a cathode flow field that directs the oxidant to flow adjacent to the cathode electrode. At about the same time a hydrogen rich reducing fluid fuel stream is directed to flow through an anode flow field that directs the fuel to flow adjacent the anode electrode. As the fuel flows through the anode flow field, a fuel-air front is created moving along the anode electrode until the fuel forces all of the air out of the anode flow field. It has been observed that the electrode that is opposite the fuel-air front experiences substantial corrosion with each start up of the known fuel cell power plant. In particular, examination of used fuel cells that experienced only a few dozen start up and shut down cycles showed that 25% to 30% of a high surface area carbon that supported the cathode catalyst of the cathode electrode had been corroded away. An explanation of a mechanism that causes that corrosion and related performance decay has been offered in a U.S. Patent Application owned by the assignee of all rights in the present invention, which Application was published on Jun. 20, 2002 under number US-2002-0076582-A1.
It is known that purging the anode and cathode flow fields with inert gases immediately upon shut down of the fuel cell power plant passivates the anode and cathode electrodes to minimize such oxidative decay. However, use of inert purge gases gives rise to substantially increased complexity and cost of the fuel cell power plant that are undesirable especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started up frequently. Another solution to the problem of start up corrosion described in the aforesaid published Application proposes an extremely rapid purging of the anode flow field upon start up with the hydrogen rich reducing fluid fuel so that air is purged from the anode flow field in no more than one second, or as quickly as no more than 0.05 seconds. It is apparent that the mechanism leading to corrosion of the carbon supporting the electrode catalyst positioned to be opposite the flow field having the fuel-air front occurs extremely rapidly during fuel cell start up. While known attempts to solve this problem have limited electrode decay, it is still desirable to eliminate or further minimize electrode corrosion upon start up of a fuel cell power plant.